The origin of anomalous eutectic-like structures in rapidly solidified eutectic Ag-Cu alloys

The formation of anomalous eutectic structures in undercooled eutectic melts has been a subject of enduring interest within the solidification community. In this study, eutectic Ag-Cu alloys were rapidly solidified via drop-tube processing, experiencing cooling rates ranging from 470 to 28000 K s−1, with the corresponding undercooling range being ≈ 20–210 K. Coarse anomalous eutectic structures were observed at eutectic cell margins in all droplets and a fine anomalous eutectic at the cell centres of droplets smaller than 300 µm. A model in which this latter high undercooling anomalous eutectic arises from the growth of a single-phase partitionless solid, followed by binodal phase separation in the solid state appears to provide a consistent explanation for the key observations relating to morphology, chemistry, crystallographic orientation and thermodynamics of the anomalous eutectic and its partitionless precursor. We find either remelting or fragmentation during recalescence are not required under the present conditions.